1. Field of the Invention
The present invention relates to an oil type lubricant to be sprayed on the occasion of forging non-ferrous metals such as aluminum, magnesium, zinc and alloys thereof or iron. Further, the present invention relates to a forging method using the oil type lubricant and to a spray apparatus.
2. Description of the Related Art
As well known, forging is a technique for deforming a metallic material to be commercialized by means of compression. This technique can be generally classified into two types, i.e., a hand forging and die forging. One good example of the hand forging may be represented by a sword which can be manufactured through the beating of an ironic material. On the other hand, the die forging is carried out by making use of a mold for homogenizing the products to be produced. One good example of the die forging is the crankshaft constituting one component of engine. Further, in order to lower the compression force required for the deformation of a metallic material, a material to be forged (hereinafter referred to as a workpiece) may be heated to soften the workpiece. The temperature for heating the workpiece may differ depending on the material constituting the workpiece. Although the forging can be classified, depending on the magnitude of heating, into cold forging, warm forging and hot forging, there is no clear numerical definition.
The cold forging is performed at a temperature of lower than the recrystallization temperature (room temperature in general) of a workpiece and the dimensional accuracy of the workpiece is very high. Accordingly, there are large possibilities that the workpiece can be commercialized without necessitating any post-work treatment. The cold forging can be suitably applied to manufactures of small products. The hot forging is performed at a temperature of higher than the recrystallization temperature of a workpiece and can be suitably applied to manufactures of large products. However, the hot forging is accompanied with problems that an oxide layer is caused to form on the surface of the workpiece and that the cracking of the product tends to be produced by the enlargement of crystal grain.
Since the metal constituting a workpiece is caused to deform in the forging, the workpiece is compressed at a high pressure. In a situation where there is no lubricant between a workpiece and a mold, galling or agglutination may occur between the workpiece and the mold. Therefore, in order to prevent these galling and agglutination, a lubricant is used for the mold.
Generally, in the case of the cold forging, a film of lubricant is more likely to be created due to the physical adsorption of the lubricant. On the other hand, in an environment of high temperatures in the hot forging, the lubricant can hardly adhere to the workpiece due to Leidenfrost's phenomenon (a kind of bumping) of lubricating components. Further, even if the lubricant is enabled to adhere to the workpiece to some extend, the absorptivity thereof is weak resulting in a difficulty in forming a firm lubricating film. In the case of the lubricant where water is employed as a medium, if the temperature of forging is lower than 100° C., water cannot be easily dried up, thereby making it difficult to form a lubricating film. However, when the temperature of forging is raised to an intermediate temperature, the lubricating film can be easily formed. Generally, lubricants to form a film can be classified into the following types.
1) Graphite film: Two kinds of lubricant film, i.e., an aqueous emulsion type and an oil type dispersion type.
2) White powder: An aqueous emulsion type of mica, boron nitride or melamine cyanurate.
3) Glass type: A mixture of colloidal silica and alkaline metal salt of aromatic carboxylic acid (Jpn. Pat. Appln. KOKAI Publication No. 60-1293), which will be diluted with water.
4) Water-soluble polymer type: Water is contained therein (Jpn. Pat. Appln. KOKAI Publication No. 1-299895).
Graphite exhibits excellent lubricity throughout temperatures ranging from low to high temperature levels. However, graphite is accompanied with a problem that the working environment will be stained with black powder, creating bad environments. Especially, in the case of a lubricant wherein graphite is mixed with oil, it would become a cause for bringing about a badly stained environment. In the case of a lubricant wherein white powder is contained as a major powder component, the working environment may not be so badly stained as compared with graphite. However, when the content of white powder is relatively large, the working site would be stained as well. Moreover, the white powder is inferior in lubricity as compared with graphite. Furthermore, if the white powder is relatively high in hardness, the surface of mold would be damaged, thus tending to shorten the useful life of the mold.
Although the glass-type and the polymer-type lubricants are useful in forming a thick film, the lubricity thereof is inferior as compared with graphite and may shorten the useful life of the mold. Furthermore, in the use of these lubricants, a glass film or a polymer film is caused to be formed on a portion around a forging apparatus, thereby necessitating a step of cleaning and hence degrading the working efficiency even though the cleaning step may not be so troublesome as in the case of the white powder.
Further, since the graphite-based and the white powder-based lubricants are dispersed in water or in oil, these lubricants are always accompanied with a problem of separation during the storage thereof or with a problem of clogging on the occasion of spraying these lubricants. In the case of water-glass-based lubricant, the dry up of the lubricant occurs in the vicinity of a spray nozzle. Especially when the interruption of work is prolonged, the dry up of the lubricant is promoted giving rise to the clogging of the nozzle. As a result, the quantity of spray would be decreased at the time of resuming the spraying work. Therefore, since the lubricating capability becomes insufficient, defective forging would result. Although the aqueous-emulsion-type lubricant is excellent in mold-cooling properties, it will necessitate a waste-water treatment.
When the inner surface of mold is heated higher than 200° C., the mist of lubricant enveloped by water layer would be boiled up on the inner surface of mold. As a result, the adhesive efficiency of the lubricant to the mold would be degraded, thus necessitating the spray of a large quantity of the lubricant. Namely, since the formation of the water-soluble lubricant film depends largely on the forging temperature, it is imperative to severely control the temperature of the mold.
Since water cannot be evaporated at a temperature lower than 100° C., the emulsion-type lubricant is unsuitable for use in the cold forging. This emulsion-type lubricant however is useful in the warm or hot forging. However, in the case of this emulsion-type lubricant, the mold is cooled by water but heated by a workpiece. When this heating/cooling cycle is repeated, cracks are generated in the mold. As a result, the mold is required to be repaired and when the number of this repair is increased, the mold which is expensive is required to be discarded. Namely, the useful life of the mold is shortened by water. Further, because the lowering of the workpiece temperature is prominent during the molding process, a high pressure molding would be required, which is one of the factors to shorten the useful life of the mold.
With respect to the spraying method, there is a problem that the cycle time is prolonged due to a large amount of spray. In the case of the water-soluble lubricant, since a large quantity of the lubricant is required to be sprayed, it is not preferable in terms of production efficiency. Additionally, due to the scattering of the lubricant resulting from a large quantity of spraying of the lubricant, there will be raised various problems such as the degrading of the working environment and the increase of frequency for replenishing the lubricant. Furthermore, the heating step of a workpiece may cause the lowering of productivity. The production process using the conventional water-soluble lubricant includes various steps after the temperature rise of the workpiece. For example, they include three steps such as a rough molding step, a finish molding step and a preliminary molding step. In this case, since the temperature of the workpiece is caused to become lower concurrent with the proceeding of molding step, the deformation resistance is caused to increase thus making it difficult to mold the workpiece. Especially, in the case of the water-soluble lubricant, since the quantity of spraying is relatively large, the mold is cooled and hence the lowering of the workpiece temperature is accelerated. In order to cope with this problem, a step of re-increasing the temperature is sometimes incorporated in the manufacturing process of the workpiece. However, the step of re-increasing the temperature leads to the increases of cycle time, working space, running cost, etc., resulting in the degrading of production efficiency.